In U.S. Pat. No. 7,077,220 (Dunn) issued Jul. 18, 2006 and assigned to the present Assignees is disclosed a hydrostatic agricultural tractor used as a self propelled swather tractor. This uses the typical construction for such tractors which has front ground wheels mounted on the frame at fixed angles parallel to each other and parallel to a center line of the frame and rear ground wheels mounted on a respective castor. Each of the front ground wheels is driven by a respective drive motor which allows variable speed in both the first and second directions such that steering of the tractor is effected by a differential in speed between the front wheels with the rear wheels following the steering in a castoring action. A pair of damper cylinders is provided, each connected between a bracket on a rear axle of the frame and a lever in the castor plane of the castor so as to provide a damping force tending to restrict rotation of the respective second wheel about the respective vertical pivot axis with the damping force at a maximum value when the rear wheels are parallel to the center line in either the forward or reverse directions.
Such hydrostatically driven tractors used primarily for swathing are commonly used and readily available. The tractor carries at a forward end on suitable sprung supports a header for cutting standing crop with the crop being transported on the header to a suitable discharge location generally centrally of the tractor for forming a swath in the field from the cut crop.
Such tractors generally include a pair of front wheels just behind the header which are fixed to the frame of the tractor so that they remain at an angle parallel to each other and parallel to a center line of the tractor. The tractor is supported at the rear end by a pair of castor wheels which are attached to a cross axle pivotally connected to the frame at a center horizontal pivot point, commonly known as a walking beam. The axle is supported relative to the ground the castor wheels at the ends of the rear axle so that the wheels trail behind a vertical pivot mount for the wheels and are free to swivel through 360° around the pivot mount.
The front wheels only are driven and are driven in a manner which allows full control in the forward and reverse directions so that steering is effected by a differential speed between the two front wheels with the rear wheels following in the conventional castoring action.
It is also known that such tractors can travel more effectively at high speed when reversed in direction so that the driven wheels are at the rear and the castor wheels at the front. The castor wheels of course rotate through 180° to trail behind the vertical pivot which is now forward of the castor wheels as the tractor moves at relatively high speed in this reversed direction.
Castor shimmy is an inherent problem on tractors of the above type. Recent machines have been increased in speed up to a maximum road speed of 24 mph instead of 16 which is the standard in the industry now. Speed aggravates the shimmy problem.
Loss of ground contact (going over bumps) also aggravates the problem and as a result weight is often added to prevent this. This results in increased power requirements and increased fuel consumption.
Up to now many such tractors use as the rear suspension of the rear castor wheels the simple pivoting rear axle or walking beam and while speeds have been relatively low, this has been acceptable. Of course the tires themselves also provide flexure which contributes to the suspension. However the use of only the tires as a suspension in rough terrain can cause sever pitching leading to significant fore and aft acceleration of the operator station.
Attempts have been made to provide a more effective suspension of the rear axle and one example is shown in U.S. Pat. No. 7,252,169 (McLean) issued Aug. 7, 2007 and assigned to CNH. This provides an arrangement in which the rear axle is split into two components each having an air spring to absorb shocks. This arrangement has obtained some acceptability but is relatively complex.
Another arrangement is shown in US Published application 2009/0174166 (Sloan et al) assigned to Deere which shows a suspension arrangement for a swather tractor which includes a frame and a rear axle rigidly carried by and substantially immovable relative to the frame. The suspension arrangement includes a caster wheel assembly having a shaft, a wheel arm, a suspension and a caster wheel. The shaft is mounted on an outboard end of the rear axle. The caster wheel is coupled with a lower end of the wheel arm. The suspension is in the form of a torsion spring including an outer tube, an inner member, and a plurality of elastomeric members interposed between the outer tube and the inner member. The outer tube is coupled with the shaft, and the inner member is coupled with an upper end of the wheel arm.